A fuel cell feeds two electrodes electrically connected to each other with a fuel and an oxidant respectively to cause electrochemical oxidation of the fuel so that the chemical energy is directly converted into electrical energy. Since it is not restricted within the Carnot cycle in contrast with the thermal power generation, high conversion efficiency of energy is exhibited. A solid polymer electrolyte fuel cell which is a fuel cell using a solid polymer electrolyte membrane as an electrolyte has advantages such as being easy in size reduction or operative at a low temperature, hence it attracts interests in application to power sources for portable or movable articles.
In the solid polymer electrolyte fuel cell, an anode runs a reaction of EQUATION(1) if hydrogen is used as the fuel.H2→2H++2e−  EQUATION(1):
Electrons generated in the EQUATION(1) flow through an external circuit to work for an external load, and thereafter reach a cathode. Protons generated in the EQUATION(1) are hydrated with water and move from the anode side, through the solid polymer electrolyte membrane, to the cathode side by electroosmosis while the protons are in the hydrated state.
On the other hand, the cathode runs a reaction of EQUATION(2) if oxygen is used as the oxidant.2H++(½)O2+2e−→H2O  EQUATION(2):
Water generated at the cathode mainly passes through a gas diffusion layer, and be exhausted outside.
Thus, the fuel cell exhausts no product except water, and it is a clean equipment for power generating.
A conventional solid polymer electrolyte fuel cell, which has mainly been developed, is one having a fuel cell stack obtained by stacking plurality of plane type cell units wherein the plane type cell unit is produced by disposing catalyst layers to be an anode and a cathode on one surface and the other surface of a plane-like shaped solid polymer electrolyte membrane respectively, further disposing gas diffusion layers on both sides of an obtained plane-like shaped membrane electrode assembly respectively, then interposing it between plane-like shaped separators.
In order to improve power density of the solid polymer electrolyte fuel cell, a proton conductive polymer membrane with a very thin membrane thickness is used as the solid polymer electrolyte membrane. Its membrane thickness is often 100 μm or less, and though a further thin electrolyte membrane is used for improvement of power density, a thickness of the cell unit can not extremely be reduced beyond conventional ones. Similarly, a catalyst layer, a gas diffusion layer, separator or the like are also undergoing their thickness reduction. However, improvement of power density per unit volume is limited even by the thickness reduction of all parts.
As the separator mentioned above, a sheet-like form carbon material which is excellent in corrosion resistance is generally used. The carbon material is expensive itself. In addition, a surface of the separator is often subjected to a fine work for forming grooves to be a gas channel in order to evenly supply the fuel gas and the oxidant gas over entire face of the plane-like membrane electrode assembly. Hence, the separator becomes too expensive due to such fine work and raises a manufacturing cost of the fuel cell.
In addition to the above described problems, the plane type cell unit has many problems such that a safe sealing of a periphery of plural cell units which are stacked in order to leak the fuel gas and the oxidant gas from the above mentioned gas channel is technically difficult, and such that the power generation efficiency is lowered due to distortion or deformation of the plane-like membrane electrode assembly.
Recently, a solid polymer electrolyte fuel cell whose basic unit of power generation is a cell module in which electrodes are disposed on a bore side and a shell side of a hollow electrolyte membrane respectively has been developed (refer to, for example, Japanese Patent Application Laid-open No. Heisei 9-223507, Japanese Patent Application Laid-open No. 2002-124273, Japanese Patent Application Laid-open No. 2002-158015 and Japanese Patent Application Laid-open No. 2002-260685).
In general, the fuel cell having such hollow-form cell modules does not need to use a member corresponding to a separator such as used in plane type. Besides, since power generation is carried out by feeding its bore and shell sides with gases of different kinds respectively, it is not particularly necessary to form gas channels. Therefore, its manufacturing process is estimated to reduce a manufacturing cost. Furthermore, since the cell module has a three dimensional form, it can make specific surface large with respect to a volume compared with a plane type cell unit, thus improvement of an output power density per unit volume can be expected.
As a method adapted to draw up a continuity from the cell, a stack comprising the plane type cell unit often employs a manner that cell units are stacked and pressed by loading a rather large pressure. Accordingly, a tight contact of the above mentioned membrane electrode assembly with the gas diffusion layer and the separator is improved by the loaded pressure, and an effective continuity is provided.
On the other hand, since the hollow-form cell module is constituted without a separator which serves as a current collector in the plane type cell unit to electrically connect cells with each other, another current collector is required.